1. Field of the Invention
The present invention relates to a load lock apparatus and method for transferring wafers between a wafer source and a processing chamber. Specifically, the present invention relates to a single or multi-wafer load lock attached directly to a process chamber in a vacuum processing system.
2. Background of Related Art
The use of cluster tools in semiconductor wafer processing is well known. Examples include the CENTURA(copyright) and ENDURA(copyright) platforms available from Applied Materials, Inc., located in Santa Clara, Calif. An example of a typical cluster tool 100 is shown in FIG. 1. Cluster tools generally include mounting a plurality of process chambers 104 to a transfer chamber 102. The transfer chamber 102 houses a centrally located robot 120 which communicates with the process chambers 104 through slit valves (not shown). Current practice includes the use of load locks 108 as intermediary chambers between pod loaders 115-118, a mini-environment 114, and the transfer chamber 102. The load lock 108 is continuously alternated between ambient pressure when communicating with pod loaders 115-118 and a vacuumed condition when communicating with the transfer chamber 102.
In operation, wafers 122 are transported from the load lock 108 into the transfer chamber 102 by the transfer chamber robot 120. Once the load lock 108 is hermetically sealed from the transfer chamber 102, a slit valve (not shown) is opened providing access between the transfer chamber 102 and a process chamber 104. The wafer 122 is transferred into the process chamber 104 where the wafer undergoes any number of processes including physical vapor deposition, chemical vapor deposition, etching, etc. During wafer transfer out of or into one load lock chamber 108, the other load lock chamber 108 can be vented to atmosphere and communicate with the pod loaders 115-118 to receive additional wafers and/or dispose of processed wafers. Cluster tools typically include two load locks to allow simultaneous communication with ambient conditions by one load lock and with vacuum conditions by the other, thereby increasing the number of wafers which can be processed by the tool. This is typically referred to as the throughput of the tool.
Efforts to achieve greater throughput are a driving force behind design changes to semiconductor manufacturing equipment. Current state of the art vacuum systems favor a highly integrated, systemic, and interdependent processing approach. One example of such a vacuum system is found in U.S. Pat. No. 5,186,718 assigned to Applied Material, Inc. of Santa Clara, Calif. In order to decrease pump-down time of various vacuum system components, additional vacuum stages are included resulting in a vacuum gradient from component to component. The objective behind such a system is to lower the pressure differential between each adjacent component which are in selective communication with one another. This approach results in reducing the time needed to pump down components such as the load lock and the processing chambers following loading of wafers into the chambers. Additionally, in order to maximize the efficiency of the system, the components are adapted to serve multiple functions.
In particular, present-day systems do not provide independent load locks which interface directly with process chambers. Rather, the load locks generally are interposed between a front-end environment and a transfer chamber. The process chambers and load locks are related by the operation of a transfer robot which shuttles wafers between the two system components. As shown in FIG. 1, the process chambers 104 share a single robot 120 so that scheduled or unscheduled interruption of the robot""s operation prevents the use of all process chambers in the cluster tool. Also, following maintenance performed on constant-vacuum components, such as the transfer chamber 102, the components must be pumped down again before processing can resume. Depending on the component, pump-down time may significantly limit throughput. A typical transfer chamber, for example, requires a pump-down time in excess of eight hours. Thus, increasing the number of constant-vacuum components drastically increases downtime and, consequently, decreases throughput.
A need therefore exists for greater independence of process chambers, fewer constant-vacuum components, and reduced operating expenses caused by manufacturing of larger systems and related maintenance of system components.
One aspect of the present invention provides a semiconductor manufacturing system having dedicated load locks for each process chamber. The load lock cycles between ambient pressure and a vacuum condition and is attached directly to the process chamber. An external robot services the load lock by transferring wafers from a wafer cassette to the load lock under atmospheric conditions. An internal load lock robot services the process chamber mounted to the load lock.
Another aspect of the present invention provides a top loading mechanism incorporated into the load lock. The loading mechanism includes a vertically movable lid connected to a motion actuator and stabilized by guide rods disposed through the lid. The external robot transfers wafers to the load lock and positions them below the raised lid and onto a wafer lifting mechanism. The wafer and the lid are then simultaneously lowered until the lid hermetically engages a sealing surface on a cover of the load lock.
The invention further provides a wafer lifting mechanism incorporated into the load lock capable of handling one or more wafers and enables the load lock to serve as a storage or cool down chamber. In one embodiment, the wafer lifting mechanism includes a plurality of lift pins disposed through the bottom of the load lock. The lift pins are selectively lowered and raised to position a wafer between the either the front-end or the processing chamber. In a second embodiment, two pairs of lift forks are disposed in the load lock to effect a transfer of storage. Each pair of forks is capable of independent vertical and rotational movement and is adapted to raise and lower a single wafer to one or more positions within the load lock.
Still another aspect of the present invention provides a transfer robot having a single degree of freedom located within the load lock. The robot includes a blade adapted to support a wafer to transfer wafers to and from a process chamber connected to the load lock. In its extended position, the robot leaves a central portion of the load lock unobstructed so that a wafer may be lowered or raised below or above the plane of movement of the robot and blade.
The invention further provides a method for transferring wafers from a load lock into a processing chamber. An atmospheric robot is provided to transfer wafers to a lifting mechanism located in a load lock. The wafer is then lowered onto an internal transfer robot centrally located in the load lock. Finally, the wafer is extended into the process chamber through a slit valve which is subsequently sealed. The wafer is retrieved by reversing these steps.